Supercritical point drying apparatus for semiconductor device manufacturing and bio-medical sample processing

ABSTRACT

A critical point drying apparatus for sample preparation in electron microscopy and semiconductor wafer production includes a computer system to automate the operational modes in drying the specimen. These operational modes controlled by the computer system are: cooling, in which a drying chamber is cooled; starting, in which the specimen chamber is filled with a transitional fluid; purging, in which the transitional fluid purges an intermediary fluid from the drying chamber and the purged internediary fluid is collected by a collector condenser; heating, in which the drying chamber is heated to elevate the transitional fluid to its critical point temperature and pressure; and bleeding, in which the drying chamber is depressurized to atmospheric pressure at a very slow rate until the drying chamber is completely vented, which signals the end of the drying operation. The drying chamber incorporates concave surfaces for pressure dispersal and to facilitate purging the intermediary fluid completely. The drying chamber incorporates angled inlet ports and windows that are sealed with gaskets. The drying chamber also incorporates the usage of a wafer holder, spacer rings and inserts to allow for the secure suspension of wafers being processed. Passing the cooling fluid through a closed loop refrigeration system may also cool the drying chamber.

CROSS-REFERENCES TO RELATED APPLICATIONS

1. This is a continuation-in-part of Application Ser. No. 09/658,185filed Sep. 8, 2000, now U.S. Pat. No. 6,493,964 the disclosure of whichis incorporated herein by reference. application Ser. No. 09/658,185 wasfiled under 35 U.S.C. § 111(a) claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dates of Provisional Application No. 60/206,726filed on May 25, 2000 pursuant to 35 U.S.C. § 111(b).

BACKGROUND OF THE PRESENT INVENTION

1. Technical Field of the Present Invention

This present invention relates to improvements in critical point dryersfor sample preparation for electron microscopy and semiconductor wafermanufacturing and especially to a computer controlled critical pointdrying apparatus. The present invention is embodied in a method forcontrolling a critical point drying apparatus using a computer system, acomputer system for implementing the method of controlling the criticalpoint drying apparatus, and a computer program product bearing softwareinstructions that implement the method of controlling the critical pointdrying apparatus.

2. Description of the Related Art

The following references provide useful background information on theindicated topics, all of which relate to the present invention, and areincorporated herein by reference:

U.S. Pat. No. 4,055,904 issued to Home on Nov. 1, 1977 describes anautomatic method of operating the purge and bleed modes for a criticalpoint dryer.

U.S. Pat. No. 4,104,808 issued to Home et al. on Aug. 8, 1978 describesa critical point dryer wherein the purge and bleed modes are controlledsemi automatically.

There will now be provided a discussion of various topics to provide aproper foundation for understanding the present invention.

In order to examine biological specimens under a scanning electronmicroscope, the biological specimens must be completely dried and coatedwith a thin conductive layer. It is important that the drying process beaccomplished without disturbing the microstructure of the biologicalspecimen to be examined. Depending upon the biological specimen'sstructure, three techniques are available for drying the biologicalspecimen. The first method is air drying by evaporation of the cellularwater. While suitable for bacteria or other rigid structures, thismethod is detrimental to the structures of many biological specimens.The surface tension forces, which turn a grape into a raisin during thedrying process, cause sufficient distortion in the cell structure ofmany biological specimens thereby rendering them useless. The secondmethod is sublimation or freeze-drying. This method is useful only forvery small specimens. Additionally, unless the lengthy technique isfollowed precisely, structural damage from thermal expansion or icecrystal formation often results. The third method utilized is the phasetransitional or critical point drying which produces consistentlyreproducible results without the drawbacks of the preceding two methods.

Along with being used to prepare specimens for the scanning electronmicroscope, critical point drying may also be used in the production ofMEMS (Micro-Electro-Mechanical Systems) devices. The critical pointdrying process helps for a sticktion free release of microstructers inthe MEMS device.

In critical point drying, a dehydrating fluid such as ethanol or acetonegradually replaces the water contained in a specimen. This maintains thethree-dimensional hydrated morphology of the structure under study.However, if the ethanol or acetone evaporates, surface tension forceswould cause structural damage and destroy the specimen's usefulness.

Critical point drying devices for sample preparation in electronmicroscopy are known in the art. The prior art critical point dryersutilize the technique of substituting a transitional fluid for thedehydrating fluid in the cell structure and then removing thetransitional fluid. A critical point dryer heats and pressurizes thebiological specimen until above the critical pressure and criticaltemperature. The critical temperature is defined as the temperatureabove which a gas cannot be liquefied by pressure alone. The criticalpressure is the pressure that results when a substance exists as a gasand a liquid in equilibrium at the critical temperature. The criticalpoint of a liquid is when its temperature and pressure are at or abovethe critical temperature and pressure and the densities of the liquidphase and vapor phase are identical. This critical point ischaracterized by an absence of phase boundaries that normally existbetween a liquid and its vapor at temperatures and pressures below thecritical point. This absence of a phase boundary eliminates the boundaryforces that exist when changing a liquid to a gas. These boundary forcesoften cause the destruction of the extremely delicate specimens whenchanging its internal liquid to a gas below the critical point.Therefore, the solution which is applied in a critical point dryingprocess is to remove the internal liquid from the biological specimenabove its critical pressure and temperature to eliminate the boundaryforce destruction that would otherwise result.

Although all fluids have a characteristic critical point which shouldallow direct removal without the use of dehydrating or transitionalfluids, the critical point temperature and pressure of water is 374.2°C. and 218 atmospheres. Achieving these temperatures and pressures wouldcause severe damage to most biological specimens and therefore a fluidhaving a lower critical temperature and pressure is normallysubstituted. Commonly, a dehydrating fluid is used that is miscible withwater (e.g., ethanol or acetone) as an intermediate stage between thespecimen containing water and a specimen containing transitional fluid.

Typically, and in the prior art dryers, the transitional fluid commonlyused is carbon dioxide (CO₂) because it is easy to use, more economical,less noxious and provides consistently better results than othertransitional fluids. The critical temperature and pressure of carbondioxide is 31° C. and 1,072 psi, respectively, thus reducing thepotential for destruction of the specimen structure.

The known instruments and apparatuses for critical point drying ofbiological specimens include, of course, a drying chamber that isconnected a supply of the transitional fluid with various regulatingvalves, temperature gauges and a means for heating the chamber. Askilled technician must carefully control the application, heating,pressurizing and removal of the transitional fluid, thus requiring notonly time but also constant attention. Applicants are unaware of acomputer-controlled critical point drying apparatus that eliminates theneed for constant operator attention.

SUMMARY OF THE PRESENT INVENTION

The present invention has been made in view of the above circumstancesand to overcome the above problems and limitations of the prior art.

Additional advantages of the present invention will be set forth in partin the description that follows and in part will be obvious from thedescription, or may be learned by practice of the present invention. Theadvantages of the present invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

According to a first aspect of the invention, a critical point dryingchamber for drying specimens comprising a chamber and at least oneheater is provided. The apparatus further comprises a first valveassembly that supplies a cooling fluid to the chamber. The apparatusfurther comprises a second valve assembly that supplies a transitionalfluid having a critical point temperature and critical point pressure tothe chamber. The apparatus further comprises a third valve assembly thatallows an intermediary fluid to be purged from the chamber. Theapparatus further comprises a fourth valve assembly that bleeds thetransitional fluid from the chamber. The apparatus further comprises acondenser collector that collects intermediary fluid exiting the thirdvalve assembly.

According to a second aspect of the invention, a critical point dryingapparatus for drying specimens comprises a cylindrical drying chamberhaving concave end portions and at least one heater. The apparatusfurther comprises a first valve assembly that supplies a cooling fluidto the drying chamber. The apparatus further comprises a second valveassembly that supplies a transitional fluid having a critical pointtemperature and critical point pressure to the drying chamber. Theapparatus further comprises a third valve assembly that allows anintermediary fluid to be purged from the drying chamber. The apparatusfurther comprises a fourth valve assembly that bleeds the transitionalfluid from the drying chamber.

According to a third aspect of the invention, a critical point dryingapparatus for drying specimens comprises a drying chamber having atleast one heater. The apparatus further comprises a first valve assemblythat supplies a cooling fluid to the drying chamber. The apparatusfurther comprises a second valve assembly that supplies a transitionalfluid having a critical point temperature and critical point pressure tothe drying chamber. The apparatus further comprises a third valveassembly that allows an intermediary fluid to be purged from the dryingchamber. The apparatus further comprises a fourth valve assembly thatbleeds the transitional fluid from the drying chamber. The apparatusfurther comprises a computer system that operates the first, second,third and fourth valve assemblies and that activates the at least oneheater to heat the transitional fluid above the critical pointtemperature and to pressurize the transitional fluid above the criticalpoint pressure.

According to a fourth aspect of the invention, a critical point dryingapparatus for drying specimens, comprising a drying chamber havingconcave end portions and at least one heater is provided. The apparatusfurther comprises a first valve assembly that supplies a cooling fluidto the drying chamber wall. The apparatus further comprises a secondvalve assembly that supplies a transitional fluid having a criticalpoint temperature and critical point pressure to the interior of thedrying chamber. The apparatus further comprises a third valve assemblythat allows an intermediary fluid to be purged from the interior of thedrying chamber. The apparatus further comprises a fourth valve assemblythat bleeds the transitional fluid from the interior of the dryingchamber at a predetermined rate. The apparatus further comprises acomputer system that operates the first, second, third and fourth valveassemblies and that activates the at least one heater to heat thetransitional fluid above the critical point temperature and topressurize the transitional fluid above the critical point pressure.

According to a fifth aspect of the invention, a critical point dryerapparatus comprising a drying chamber and a computer system wherein thecomputer system is adapted to controlling the drying chamber during acritical point drying process is provided. The computer system comprisesa processor and a memory comprising software instructions adapted toenable the computer system. The software instructions enable thecomputer system to cool the drying chamber to a first chambertemperature. The software instructions further enable the computersystem to fill the drying chamber with a transitional fluid having acritical point temperature and critical point pressure while maintainingthe drying chamber at the first chamber temperature such that thetransitional fluid completely displaces an intermediary fluid within afirst time period. The software instructions further enable the computersystem to activate at least one heater to raise the transitional fluidto its critical point pressure and critical point temperature, therebyreaching critical point equilibrium. The software instructions fartherenable the computer system to maintain the transitional fluid at thecritical point equilibrium for a second time period. The softwareinstructions further enable the computer system to bleeding thetransitional fluid from the drying chamber while maintaining the dryingchamber at the second chamber temperature and allowing the dryingchamber pressure to drop.

According to a sixth aspect of the invention, a computer program productfor enabling a computer system to control the drying chamber during acritical point drying process is provided. The computer program productcomprises software instructions for enabling the computer system toperform predetermined operations, and a computer readable medium bearingthe software instructions. The predetermined operations comprise coolingthe drying chamber to a first chamber temperature. The predeterminedoperations comprise filling the drying chamber with a transitional fluidhaving a critical point temperature and critical point pressure whilemaintaining the drying chamber at the first chamber temperature suchthat the transitional fluid completely displaces the intermediary fluidwithin a first time period. The predetermined operations compriseactivating at least one heater to raise the transitional fluid to itscritical point pressure and critical point temperature, thereby reachingcritical point equilibrium. The predetermined operations comprisemaintaining the transitional fluid at the critical point equilibrium fora second time period. The predetermined operations comprise bleeding thetransitional fluid from the drying chamber while maintaining the dryingchamber at the second chamber temperature.

According to a seventh aspect of the invention, an article ofmanufacture, which comprises a computer readable medium having storedtherein a computer program to control a drying chamber during a criticalpoint drying process, is provided. The article of manufacture comprisesa first code segment which, when executed on a computer, cools thedrying chamber to a first chamber temperature. The article ofmanufacture comprises a second code segment which, when executed on acomputer, fills the drying chamber with a transitional fluid having acritical point temperature and critical point pressure while maintainingthe drying chamber at the first chamber temperature such that thetransitional fluid completely displaces the intermediary fluid within afirst time period. The article of manufacture comprises a third codesegment which, when executed on a computer, activates at least oneheater to raise the transitional fluid to its critical point pressureand critical point temperature, thereby reaching a critical pointequilibrium. The article of manufacture comprises a fourth code segmentwhich, when executed on a computer, maintains the transitional fluid atthe critical point equilibrium for a second time period. The article ofmanufacture comprises a fifth code segment which, when executed on acomputer, bleeds the transitional fluid from the drying chamber whilemaintaining the drying chamber at the second chamber temperature.

According to an eighth aspect of the invention, a critical point dryingchamber for drying specimens, comprising a chamber with a heater isprovided. The critical point drying apparatus further comprises a firstvalve assembly that supplies a cooling fluid to the chamber, and asecond valve assembly that supplies a transitional fluid having acritical point temperature and critical point pressure to the chamber.The critical point drying chamber further comprises a third valveassembly that allows an intermediary fluid to be purged from thechamber, and that bleeds the transitional fluid from the chamber, and acondenser collector that collects intermediary fluid exiting the thirdvalve assembly. The drying chamber further comprises a wafer holder, anda chamber insert to change the inner diameter of the drying chamber.

According to a ninth aspect of the invention, a critical point dryingchamber for drying specimens, comprising a chamber with a heater isprovided. The drying chamber is a cylindrical drying chamber havingconcave end portions and a first valve assembly that supplies a coolingfluid to the drying chamber. The critical point drying apparatus furthercomprises a second valve assembly that supplies a transitional fluidhaving a critical point temperature and critical point pressure to thedrying chamber, and a third valve assembly that allows an intermediaryfluid to be purged from the drying chamber; and that bleeds thetransitional fluid from the drying chamber. The drying chamber furthercomprises a wafer holder, and a chamber insert to change the innerdiameter of the drying chamber.

According to a tenth aspect of the invention, a critical point dryingchamber for drying specimens, comprising a chamber with a heater isprovided. The critical point drying chamber further comprises a firstvalve assembly that supplies a cooling fluid to the drying chamber, asecond valve assembly that supplies a transitional fluid having acritical point temperature and critical point pressure to the dryingchamber, and a third valve assembly that allows an intermediary fluid tobe purged from the drying chamber, and that bleeds the transitionalfluid from the drying chamber. The critical point drying apparatusfurther comprises a computer system that operates the first, second andthird valve assemblies and that activates the heater to heat thetransitional fluid above the critical point temperature and topressurize the transitional fluid above the critical point pressure. Thedrying chamber further comprises a wafer holder, and a chamber insert tochange the inner diameter of the drying chamber.

According to an eleventh aspect of the invention, a critical pointdrying chamber for drying specimens, comprising a chamber with a heateris provided. The drying chamber is a cylindrical drying chamber havingconcave end portions and a first valve assembly that supplies a coolingfluid to the drying chamber. The critical point drying apparatus furthercomprises a second valve assembly that supplies a transitional fluidhaving a critical point temperature and critical point pressure to thedrying chamber, and a third valve assembly that allows an intermediaryfluid to be purged from the drying chamber, and that bleeds thetransitional fluid from the drying chamber. In addition, the criticalpoint drying apparatus furtherer comprises a computer system thatoperates the first, second and third valve assemblies and that activatesthe heater to heat the transitional fluid above the critical pointtemperature and to pressurize the transitional fluid above the criticalpoint pressure. The drying chamber further comprises a wafer holder, anda chamber insert to change the inner diameter of the drying chamber.

The above advantages of the present invention will become apparent fromthe following detailed description and with reference to theaccompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate embodiments of the presentinvention and, together with the written description, serve to explainthe advantages and principles of the present invention. In the drawings,

FIG. 1 illustrates a perspective view of a first embodiment of theinvention;

FIG. 2 illustrates a perspective view of a second embodiment of theinvention;

FIG. 3 is a top view of the drying chamber illustrating the location ofthe purge outlets, the fill inlets, the cool inlet and the cool outlet;

FIG. 4 is a top view of the chamber cover;

FIG. 5 illustrates a cross-sectional view along lines IV—IV of thechamber cover, the cover viewing port and the cover viewing window;

FIG. 6 illustrates a cross-sectional view of the drying chamber alonglines I—I showing the fill inlets, the purge outlets, the chamberviewing port and the chamber viewing window;

FIG. 7 illustrates a cross-sectional view of the drying chamber alonglines II—II showing the temperature sensor, the heat sensor, the coolsensor and the heater;

FIG. 8 illustrates a cross-sectional view of the drying chamber alonglines IlI—III showing the cool inlet and the cool outlet;

FIG. 9 illustrates the various valves and connection lines for therouting of cooling fluid and transitional fluid through the criticalpoint drying apparatus;

FIG. 9A illustrates another embodiment of the various valves andconnection lines for the routing of cooling fluid and transitional fluidthrough the critical point drying apparatus;

FIG. 10 illustrates the various valves and connection lines for therouting of cooling fluid and transitional fluid through a critical pointdrying apparatus that is connected to a closed loop refrigerationsystem;

FIG. 10A illustrates another embodiment of the various valves andconnection lines for the routing of cooling fluid and transitional fluidthrough a critical point drying apparatus that is connected to a closedloop refrigeration system;

FIG. 11 illustrates a side view of the collector condenser;

FIG. 12 illustrates the data flow paths between the computer system andthe various valves and sensors;

FIG. 13A illustrates a view of the spacer ring;

FIG. 13B illustrates a side view of the wafer holder,

FIG. 13C illustrates a side view of the chamber insert for the dryingchamber;

FIG. 14 illustrates the process steps executed by the computer systemwhen controlling a critical point drying process in the drying chamber;

FIGS. 15A-15F illustrate the process steps, in more detail, executed bythe computer system when controlling a critical point drying processwithin the drying chamber; and

FIG. 16 is a table showing the valve openings and closings for eachdrying mode controlled by the computer system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Prior to describing the aspects of the present invention, somedetails concerning certain terms of art will be provided to facilitatethe reader's understanding of the present invention and to set forth themeaning of various terms.

As used herein, the term “computer system” encompasses the widestpossible meaning and includes, but is not limited to, microprocessors,standalone processors, networked processors, mainframe processors, andprocessors in a client/server relationship. The term “computer system”is to be understood to include at least a memory and a processor. Ingeneral, the memory will store, at one time or another, at leastportions of executable program code, and the processor will execute oneor more of the instructions included in that executable program code.

As used herein, the term “embedded computer system” includes, but is notlimited to, an embedded central processor and memory bearing object codeinstructions. Examples of embedded computer systems include, but are notlimited to, personal digital assistants, cellular phones and digitalcameras. In general, any device or appliance that uses a centralprocessor, no matter how primitive, to control its functions can belabeled has having an embedded computer system. The embedded centralprocessor will execute one or more of the object code instructions thatare stored on the memory. The embedded computer system can include cachememory, input/output devices and other peripherals.

It will be appreciated that the term “predetermined operations” and theterm “computer system software” mean substantially the same thing forthe purposes of this description. It is not necessary to the practice ofthis present invention that the memory and the processor be physicallylocated in the same place. That is to say, it is foreseen that theprocessor and the memory might be in different physical pieces ofequipment or even in geographically distinct locations.

As used herein, one of skill in the art will appreciate that “media” or“computer-readable media” may include a diskette, a tape, a compactdisc, an integrated circuit, a cartridge, a remote transmission via acommunications circuit, or any other similar medium useable bycomputers. For example, to distribute computer system software, thesupplier might provide a diskette or might transmit the instructions forperforming predetermined operations in some form via satellitetransmission, via a direct telephone link, or via the Internet.

Although computer system software might be “written on” a diskette,“stored in” an integrated circuit, or “carried over” a communicationscircuit, it will be appreciated that, for the purposes of thisdiscussion, the computer usable medium will be referred to as “bearing”the instructions for performing predetermined operations. Thus, the term“bearing” is intended to encompass the above and all equivalent ways inwhich instructions for performing predetermined operations areassociated with a computer usable medium.

Therefore, for the sake of simplicity, the term “program product” ishereafter used to refer to a computer useable medium, as defined above,which bears instructions for performing predetermined operations in anyform.

A detailed description of the preferred embodiments of the presentinvention will now be given referring to the accompanying drawingswherein like reference numerals refer to similar parts in the severalviews.

The general operation of a critical point dryer will be provided tofacilitate the reader's understanding of the present invention. Thisdescription of the general operation of a critical point dryer is by nomeans limiting on the operation of the present invention. The first stepin a critical point drying operation is to cool the drying chamber to atemperature that will condense the transitional fluid to be added later.A cooling fluid flows around the drying chamber and evaporates in a heatexchange relationship with the drying chamber. Preferably, the coolingfluid is liquid carbon dioxide. If a closed loop refrigeration system isused to cool the drying chamber, Freon™ or its equivalent can also beused. Next, the treated specimen is placed in the drying chamber alongwith an amount of the dehydrating fluid, such as ethanol, methanol oracetone. The specimen has previously been dehydrated with thedehydrating fluid. After the treated specimen has been placed in thedrying chamber and the cover secured, a transitional fluid fills thedrying chamber. After the drying chamber is filled, the dehydratingfluid is purged from the treated specimen in the drying chamber.Preferably, the transitional fluid is liquid carbon dioxide. The purgingof the dehydrating fluid is controlled over a predetermined time period.In an aspect of the invention, the critical point drying apparatuscycles through another filling of the drying chamber with thetransitional fluid to ensure that the transitional fluid completelyfills the drying chamber. The drying chamber is then heated to raise thetransitional fluid to its critical point pressure and critical pointtemperature, thereby reaching critical point equilibrium. Once criticalpoint equilibrium is reached, the equilibrium is maintained for acertain length of time. After the equilibrium time period has expired,the drying chamber temperature is maintained while the drying chamberpressure is reduced or bled-off very slowly. When the drying chamberpressure drops below a predetermined threshold, the drying chamber isthen vented to release any residual pressure that may hinder removal ofthe chamber cover and the chamber heat is shut off.

Referring to FIG. 1, a perspective view showing the externalconfiguration of an embodiment of the critical point drying apparatus 1is shown. The housing 24 encloses the internal valves, wiring, piping,switches, relays and computer system components that make up thecritical point drying apparatus 1. A power switch 22 applies electricalpower to the dryer through a fuse. The operation indicator lightsindicate the individual operation that is being undertaken in the dryingchamber 40. Preferably, the operation indicators are light emittingdiodes (LED). The critical point drying apparatus 1 has the followingoperation indicators: cool LED 16, fill LED 17, purge LED 18, heat LED19, bleed LED 20 and vent LED 21. The temperature gauge 23 and thepressure gauge 69 provide visible indicators of the present conditionswithin the drying chamber 40. Transitional fluid and cooling fluid enterthe critical point drying apparatus 1 through the inlet port 52.Exhausted cooling fluid exits the critical point drying apparatus 1through the cool exit port 51. Purged dehydrating fluid and exhaustedtransitional fluid exits the critical point drying apparatus 1 throughpurge port 50.

Referring to FIG. 2, a perspective view showing the externalconfiguration of another embodiment of the critical point dryingapparatus 1′ is shown. The critical point drying apparatus 1′ is shownwith a cool supply port 71. The cool supply port 71 is connected to aclosed loop refrigeration system. Cooling fluid circulates through thecritical point drying apparatus 1′ by entering the cool supply port 71and exiting through the cool exit port 51. Transitional fluid enters thecritical point drying apparatus 1′ through the inlet port 52. Purgeddehydrating fluid and exhausted transitional fluid exits the criticalpoint drying apparatus 1′ through purge port 50.

Referring to FIG. 3, a top view of the drying chamber 40 is illustrated.The placement of the various inlets and outlets shown in FIG. 3 is by nomeans limiting and is shown for illustration purposes only. Thecross-sectional view along lines I—I includes the fill inlets and thepurge ports and is shown in more detail in FIG. 6. The cross-sectionalview along lines II—II includes the heater and is shown in more detailin FIG. 7. The cross-sectional view along lines III—III includes thecool inlet and the cool outlet and is shown in more detail in FIG. 8. Asshown in FIG. 3, the fill inlets 43,44 have an angled portion 39 that isangled relative to the drying chamber wall 47 such that when thetransitional fluid enters the drying chamber 40, it will flow into thedrying chamber 40 in a swirling fashion. The swirling of thetransitional fluid as it enters the drying chamber 40 allows for an evenand thorough purge of the intermediary fluid from the drying chamber 40.

Referring to FIG. 4, a top view of the chamber cover 75 is illustrated.Cross-section IV—IV includes the mounting stud holes 76 and is shown inmore detail in FIG. 5. The chamber cover 75 has a cover viewing port 80for observing the interior of the drying chamber 40. Preferably, themounting stud holes 76 are evenly arranged around the perimeter of thechamber cover 75.

Referring to FIG. 5, the cross-section of the chamber cover 75 alonglines IV—IV is illustrated in more detail. This is the cross-sectionalong lines IV—IV referred to by FIG. 4. The chamber cover 75 secured bysecuring knobs (not shown) and is provided with a cover viewing window78 for viewing the specimen during the operation of the critical pointdrying apparatus 1.

The cover viewing window 78 is mounted on the axial center of thechamber cover 75. Preferably, the cover viewing window 78 comprisesquartz or an equivalent material. The operator can view the interior ofthe drying chamber 40 through the cover viewing port 80 in the chambercover 75 and monitor the progress of the critical point drying sequence.The cover viewing window 78 is mounted with a precision machined viewingwindow gasket 77 that is pressure fit into the cover viewing port 80 inthe chamber cover 75. Preferably, the viewing window gasket 77 comprisesTeflon™ or an equivalent material. The viewing window gasket 77 holdsthe cover viewing window 78 in place, thereby providing a seal thatwithstands high pressure and will not be damaged by the intermediaryfluids.

In an aspect of the invention, the chamber cover 75 has a concavesurface 79 on its underside that has a concave pitch. The concave pitchacts to evenly displace the internal pressure of the drying chamber 40exerted on the chamber cover 75. The concave surface 79 allows thecritical point drying apparatus 1 to accommodate large specimens withoutwarping the chamber cover 75. For example, wafers used in integratedcircuit manufacturing can be upwards of twelve inches in diameter. Aconventional chamber cover would likely warp and be difficult to removefrom the drying chamber.

Referring to FIG. 6, the cross-section of the drying chamber 40 alonglines I—I will be described in more detail. The drying chamber 40 isalso provided with a fill inlets 43,44 and a purge outlets 45,46 thatallow the transitional fluid to fill and flow through the chamber. Asthe size (i.e. diameter) of the drying chamber 40 increases, the fillinlets 43,44 and the purge outlets 45,46 serve to increase theefficiency of the critical point drying apparatus 1. The fill inlet43,44 has an angled portion 39 that is angled relative to the dryingchamber wall 47 such that when the transitional fluid enters the dryingchamber 40, it will flow into the drying chamber 40 in a swirlingfashion. The swirling of the transitional fluid as it enters the dryingchamber 40 allows for an even and thorough purge of the intermediaryfluid from the drying chamber 40. A small chamber may require only asingle fill inlet 43 and a single purge outlet 45 to adequately flow thetransitional fluid through the drying chamber 40. However, as dryingchamber size increases, a plurality of fill inlets 43,44, are used andall have an angled portion 39 that is arranged at an angle relative tothe drying chamber wall 47, which efficiently circulates the incomingtransitional fluid. An added benefit of multiple fill inlets is that thedrying chamber 40 fills at a much faster rate, thereby inducing lessdisturbance to the specimen situated in the drying chamber 40.Similarly, a large drying chamber has a plurality of purge outlets 45,46situated at the lowest point of the drying chamber 40 that aids in thecollection and purging of the intermediary fluid.

The drying chamber 40 has a chamber viewing window 114 mounted in theaxial center of the drying chamber 40. Preferably, the chamber viewingwindow 114 comprises quartz or an equivalent material. The chamberviewing window 114 is lighted from below and the operator can view theinterior of the drying chamber 40 through the cover viewing port 80 inthe chamber cover 75 and monitor the progress of the critical pointdrying sequence. The chamber viewing window 114 is mounted with aprecision machined viewing window gasket 115 that is pressure fit into achamber viewing port 116 in the bottom of the drying chamber 40.Preferably, the viewing window gasket 115 comprises Teflon™ or anequivalent material. The viewing window gasket 115 holds the chamberviewing window 114 in place, thereby providing a seal that withstandshigh pressure and will not be damaged by the intermediary fluids.

Mounting studs 110 extend upward through the chamber cover 75 and inconjunction with internally threaded securing knobs (not shown), fixablyand sealably mount the chamber cover 75 to the drying chamber 40. Bydrilling through the drying chamber wall 47 into the mounting studs 110,stainless steel mounting pins 111 can be inserted into the mountingstuds 110 to prevent any movement.

The bottom of the drying chamber 40 has a chamber concave surface 48with a concave pitch. The concave pitch acts to each active to evenlydisplace the internal pressure of the drying chamber 40. The chamberconcave surface 48 allows the intermediary fluid to collect and exitthrough the purge ports 45,46 that are located at the lowest point ofthe drying chamber. The concavity thus assists with dispersing theinternal pressure and with the complete purging of the intermediaryfluid through the bottom of the drying chamber 40. The chamber concavesurface 48 allows the critical point drying apparatus 1 to accommodatelarge specimens without warping in the chamber cover 75.

A cover gasket 113 provides a seal between the chamber cover 75 and thetop portion of the drying chamber 40. A seal groove 112 is formed in thetop portion of the drying chamber 40 that directly opposes the chambercover 75. The cover gasket 113 is set into the seal groove 112, andprojects slightly above the top surface of the drying chamber 40,thereby ensuring a tight seal when the chamber cover 75 is secured. Dueto the pressure in the drying chamber 40, the cover gasket 113 must bemade from a material that is inert to the fluids used in the dryingchamber 40 and must be able to withstand the chamber pressure withoutdeformation. Preferably, the cover gasket 113 comprises Teflon™ or anequivalent material.

Referring to FIG. 7, the cross-section of the drying chamber 40 alonglines II—II is illustrated in more detail. The drying chamber 40 alsohas at least one heater 33 mounted in a cavity 34 in the drying chamberwall 47 to heat the transitional fluid above the critical point.Preferably, the heater 33 is a wirewound resistance heater that iscontrolled by the computer system 99.

A temperature sensor (not shown) is mounted in the wall of the dryingchamber 40 is connected to a temperature gauge 23 to provide anindication of the drying chamber temperature. In addition, three othertemperature sensors are mounted on the drying chamber wall 47. A heatsensor 31 (normally closed) opens when the temperature in the dryingchamber 40 reaches a predetermined level and the opening of the heatsensor 31 is monitored by the computer system 99. Preferably, the beatsensor 31 is a thermostatic sensor and opens when the drying chambertemperature exceeds 42° C. A cool sensor 32 (normally closed) opens whenthe temperature in the drying chamber 40 drops past a predeterminedlevel and the opening of the cool sensor 32 is monitored by the computersystem 99. Preferably, the cool sensor 32 is a thermostatic sensor andopens when the drying chamber temperature is less than 5° C. Finally,the safety sensor 30 ensures that the heater 33 does not raise thedrying chamber temperature past a predetermined safety level. If thedrying chamber temperature exceeds the predetermined safety level, allpower to the heater 33 is cut off. Preferably, the safety sensor 30 is athermostatic sensor and opens when the drying chamber temperatureexceeds 50° C.

Referring to FIG. 8, the cross-section of the drying chamber 40 alonglines III—III is illustrated in more detail. A cooling fluid iscirculated to cool the drying chamber wall 47 by passing into the coolinlet 41 through the walls of the drying chamber and the connecting line49 and out through the cool outlet 42. Preferably, liquid carbon dioxideis used as a cooling fluid. The cooling fluid cools the drying chamber40 by adiabatic cooling, which is turned on and off automatically viathermostatic controls. The fittings and tubing through which the coolingfluid flows is preferably stainless steel. In order to protect theintegrity of the specimen, all external and internal surfaces of thecritical point drying apparatus 1 are both chemically and moistureresistant, and all internal surfaces are inert to the intermediary andtransitional fluids, such as liquid carbon dioxide and ultra-purealcohol. In addition, all internal and external surfaces are grounded toguard against static discharge that is harmful to semiconductor wafers.

Referring to FIG. 9, a schematic diagram showing the flow of fluidsthrough the drying chamber 40 and the various control valves isillustrated. The transitional fluid is also utilized to cool the dryingchamber 40 through the cooling circuit shown. The transitional fluid,preferably liquid carbon dioxide, is provided at the inlet port 52 andthen flows through a filter assembly 53 to the 3-way tee 54. The filterassembly 53 removes any particulate matter from the transitional fluidprior to entering the various valves of the critical point dryingapparatus 1. Preferably, the filter assembly 53 is a stainless steelfilter that removes particulate up to 0.5 microns in diameter in orderto protect the specimen in the drying chamber 40, as well as thecritical point drying apparatus valves. Preferably, all tubing,fittings, valves, etc. are stainless steel. In addition, a rupture discthat will burst and evacuate the system in case of an undesirable risein pressure protects the entire pressure system (not shown).

The transitional fluid is then piped to a pair of computer systemcontrolled solenoid operated valves: the fill valve 56 and the coolvalve 55. The fill valve 56 and cool valve 55 also comprise meteringvalves to control the flow rate of fluids through the valves.

When the computer system 99 energizes the cool valve 55 solenoid, thecool valve 55 supplies the transitional fluid through the metering valveto the drying chamber 40 at the cool inlet 41. The metering valveregulates the flow of transitional fluid through the wall of the dryingchamber. When the cool valve 55 is energized, the transitional fluidflows out from the cool valve 55 and through the connection line to thedrying chamber 40, wherein the transitional fluid is evaporated andducted throughout the wall of the drying chamber 40. The flow is fromthe cool inlet 41 through the wall of the drying chamber 40 to the cooloutlet 42. The warmed vaporized cooling fluid is ducted out of thecritical point drying apparatus 1 at the cool port 51. If a closed looprefrigeration system is used, the cooling fluid is cycled back to arefrigeration unit. The closed loop system may use a refrigerant otherthan liquid carbon dioxide, such as Freon™.

When the computer system 99 energizes the fill valve 56, transitionalfluid flows into the fill inlets 43,44 to fill or purge the dryingchamber 40. When the drying chamber 40 is being filled with thetransitional fluid, the computer system 99 energizes the fill valve 56,and transitional fluid flows from the fill valve 56, through a checkvalve 57, a 4-way tee 58 and into the drying chamber 40 through the fillinlets 43,44. The fill inlets 43,44 are coupled to each other through4-way tee 58 and 3-way tee 59. The in-line check valve 57 protects thefill valve from any backflow to the drying chamber.

A connection line from the 4-way tee 58 is connected to a high-pressuresensor 67, a low-pressure sensor 68 and a pressure gauge 69. Thelow-pressure sensor 68 opens when the chamber pressure drops below apredetermined low pressure point, and the opening of the low-pressuresensor 68 is monitored by the computer system 99. An acceptable rangefor the predetermined low pressure point is from 100 to 600 psi.Preferably, the predetermined low pressure point is 400 psi.

The high-pressure sensor 67 opens when the chamber pressure exceeds apredetermined high pressure point, and the opening of the high-pressuresensor 67 is monitored by the computer system 99. An acceptable rangefor the predetermined high pressure point is from 1175 and 1600 psi.Preferably, the predetermined high pressure point is 1200 psi.

A pressure relief valve (not shown) is also connected to the pressuresensors. The pressure relief valve will release the pressure from thecritical point drying apparatus 1 when the drying chamber pressureexceeds a predetermined limit. The pressure relief valve is heated andthe heating of the pressure relief valve is thermostatically controlledindependent of the computer system 99. An acceptable range for thepressure relief valve is from 1200 to 1600 psi. Preferably, the pressurerelief valve opens at 1250 psi.

A rupture disc 72 is also connected to the pressure sensors. The rupturedisc 72 acts as an additional safety feature and is set to burst whenthe drying chamber pressure goes above a predetermined limit. Anacceptable range for the rupture disc 72 is from 1900 to 3000 psi.Preferably, the rupture disc 72 ruptures at 2100 psi.

When the intermediary fluid is to be purged from the drying chamber 40,the computer system 99 commands the purge valve 63 to open. The purgevalve 63 is a solenoid valve. The intermediary fluid is forced from thedrying chamber 40 through the purge outlets 45,46 in the drying concavesurface 48 and into the connection line through a filter assembly 61,which is connected to the purge valve 63. The purge valve 63 is heatedto prevent the purge valve 63 from freezing when the transitional fluidor the intermediary fluid passes through the purge valve 63. The heatingof the purge 63 valve is thermostatically controlled independent of thecomputer system 99. The purge valve 63 also includes a metering valve tocontrol the flow rate at which the transitional fluid or intermediaryfluid is purged. Attached to the exiting end of the purge metering valveis a check valve 65 to prevent fluid backflow through the purge valve 63into the drying chamber 40. The optional check valve 65 is connected tothe purge outlet 50 by 3-way tee 66. The purged fluid exits the criticalpoint drying apparatus 1 through the purge outlet 50. Throughout theentire purging process, the computer system 99 monitors the dryingchamber temperature and keeps the drying chamber 40 below apredetermined temperature, preferably 5° C. or less.

The cycle time for executing a purge of the intermediary fluid from thedrying chamber 40 is controlled by the computer system 99. Preferably,the purge time is adjusted by a purge timing control 25 that is locatedon the housing 24 of the critical point drying apparatus 1. After thepurge cycle for the purging of the intermediary fluid is complete, thecomputer system 99 closes the purge valve 63 and allows the fill valve56 to continue filling the drying chamber 40 with transitional fluid.This ensures the transitional fluid fills the drying chamber 40completely. The computer system 99 then advances the drying chamber 40into the heating cycle.

The computer system 99 activates the heater 32 to raise the transitionalfluid to its critical point pressure and critical point temperature,thereby reaching critical point equilibrium. Preferably, the heater 32raises the drying chamber temperature to at least 311° C. or greater,which, in turn, causes the temperature and pressure of the transitionalfluid to reach its critical point temperature and pressure.

After the computer system 99 has determined that the specimen has beenat the critical point equilibrium for a sufficient amount of time, thecomputer system 99 commands the bleed valve 62 to open, thereby allowingthe transitional fluid to exhaust out of the drying chamber 40 and exitthe critical point drying apparatus 1 through the purge outlet 50. Thebleed valve 62 is a solenoid valve. When the transitional fluid isexhausted, it flows from the drying chamber 40 through a filter assembly60 into the bleed valve 62 and then into the check valve 64. The checkvalve 64 prevents backflow from backing through the bleed valve 62 intothe drying chamber 40. The optional check valve 64 is connected to thepurge outlet 50 through 3-way tee 66. The bleed valve 62 also comprisesa metering valve to control the bleed rate. Preferably, the meteringvalve allows the system pressure to decrease at a rate of 100psi/minute. This bleed rate prevents the transitional fluid fromrecondensing. In addition, the bleed valve 62 is thermostatically heatedto prevent the bleed valve 62 from freezing as the transitional fluidflows through it. During the bleed process, the computer system 99maintains the drying chamber temperature at 31° C. or above. Thistemperature level prevents recondensation on the specimen. When thedrying chamber pressure is reduced to 400 psi, the computer system 99turns off the heater 32 and switches from bleed to vent mode. Thecomputer system 99 commands the bleed valve 62 to close and the purgevalve 63 to open. This returns the drying chamber to atmosphericpressure quicker.

Referring to FIG. 9A, a schematic diagram showing the flow of fluidsthrough the drying chamber 40 and the various control valves isillustrated. This is an embodiment of the present invention that usesonly three valves to control the flow of fluids through the dryingchamber 40. In the embodiment depicted in FIG. 9A, the filter assembly60, the bleed valve 62, the check valve 64 and the three-way tee 66 areeliminated. The purge port 50 is connected to the purge valve 63 throughthe check valve 65. The use of the check valve 65 is optional. The fillinlet 44 is directly connected to the 4-way tee 58. After the computersystem 99 has determined that the specimen has been at the criticalpoint equilibrium for a sufficient amount of time, the computer system99 conmmands the purge valve 63 to open, thereby allowing thetransitional fluid to exhaust out of the drying chamber 40 and exit thecritical point drying apparatus 1 through the purge outlet 50. The purgevalve 63 is thermostatically heated to prevent the purge valve 63 fromfreezing as the transitional fluid flows through it. During the bleedprocess, the computer system 99 maintains the drying chamber temperatureat 31° C. or above. This temperature level prevents recondensation onthe specimen. When the drying chamber pressure is reduced to 400 psi,the computer system 99 turns off the heater 32 and switches from bleedto vent mode.

Referring to FIG. 10, in another aspect of the invention, the dryingchamber 40 can be cooled using a closed loop refrigeration system. Theclosed loop system is coupled to the drying chamber 40 and allows thedrying chamber 40 to be cooled without blasting liquid carbon dioxidethrough the drying chamber 40. This feature of the present inventionprovides for savings on the amount of carbon dioxide that is used duringa specimen drying sequence. In addition, the closed loop system may usea refrigerant other than liquid carbon dioxide, such as Freon™. Coolingfluid enters the critical point drying apparatus 1 through cool supplyport 71 and is piped to the cool valve 55. A common supply for coolingfluid and transitional fluid is not used.

Referring to FIG. 10A, in another embodiment of the present invention,the drying chamber 40 can be cooled using a closed loop refrigerationsystem with only three valves to control the flow of fluids through thedrying chamber 40. In the embodiment depicted in FIG. 10A, the filterassembly 60, the bleed valve 62, the check valve 64 and the three-waytee 66 are eliminated. The purge port 50 is connected to the purge valve63 through the check valve 65. The use of the check valve 65 isoptional. The fill inlet 44 is directly connected to the 4-way tee 58.After the computer system 99 has determined that the specimen has beenat the critical point equilibrium for a sufficient amount of time, thecomputer system 99 commands the purge valve 63 to open, thereby allowingthe transitional fluid to exhaust out of the drying chamber 40 and exitthe critical point drying apparatus 1 through the purge outlet 50. Thepurge valve 63 is thermostatically heated to prevent the purge valve 63from freezing as the transitional fluid flows through it. During thebleed process, the computer system 99 maintains the drying chambertemperature at 31° C. or above. This temperature level preventsrecondensation on the specimen. When the drying chamber pressure isreduced to 400 psi, the computer system 99 turns off the heater 32 andswitches from bleed to vent mode.

Referring to FIG. 11, a condenser collector 130 allows the intermediaryfluid (i.e., the dehydrating fluid) to be purged from the drying chamber40 in a safer and more controlled manner. The condenser collector 130 isconnected to the purge port 50. Preferably, methyl alcohol or ethylalcohol is used as intermediary fluid for the specimen drying process,although these fluids are relatively toxic. When the intermediary fluidis purged from the drying chamber 40 under pressure, the intermediaryfluid often exits the drying chamber 40 in a frozen state. The condensercollector 130 transforms the intermediary fluid back into a liquid stateso that it may be drained from the condenser collector 130 easily.Preferably, the condenser collector 130 uses a thermostaticallycontrolled heated reservoir 131 with a concave bottom portion 133. Thedrain line 134 the condenser collector 130 is located on the low pointof the concave bottom portion 133 and a drain valve 135 regulates theexit flow of the material.

The outgoing intermediary fluid, both in its gaseous state and frozenstate, enters through a reservoir inlet 137 on the upper portion of thereservoir 131. The gas is allowed to exit through the gas vent 136 ofthe reservoir 131, while the frozen portions of the intermediary fluidremain in the reservoir 131. The heater 132 then warms the reservoir 131and the frozen intermediary fluid returns to a liquid state and is thendrained from the reservoir 131. Preferably, the condenser collector 130can be constructed from stainless steel, brass or polyvinyl chloride.The reservoir 131 of the condenser collector 130 is insulated to preventcondensation from developing on the surfaces of the reservoir 131.

Referring to FIG. 12, the computer system 99 with software that isadapted to perform critical point drying techniques when coupled to acritical point drying apparatus 1 will now be described in summaryfashion. As described above, the computer system 99 includes a processorfor executing software instructions adapted to enable the computer tocontrol a critical point drying chamber and its associated valve andheaters. The computer system 99 includes a memory 101 that stores thesoftware instructions adapted to enable the computer system 99 tocontrol the drying chamber 40 and associated valves. The computer system99 also comprises an I/O port that allows the computer system 99 to bere-programmed and to upload/download data. As shown in FIG. 12, thecomputer system 99 receives inputs from the temperature and pressuresensors through the data lines 103. Commands to open or close the coolvalve 55, the fill valve 56, the bleed valve 62 and the purge valve 63are sent over the data lines 103. Finally, the computer system 99receives commands from the indicator switches on the housing 24 thatindicate the various modes, and the computer system 99 also lights theappropriate indicator switch to inform the operator which mode iscurrently being executed.

Referring to FIGS. 13A-13C, when the critical point drying apparatus 1will be used to process semiconductor wafers, the drying chamber must beoutfitted with a mechanism to hold the semiconductor wafer in asuspended position so that the drying process can be achieved. Thecritical point drying apparatus 1 that are designed to processsemiconductor wafers are outfitted with a wafer holder 124, a spacerring 120 and a chamber insert 121. Preferably, these components come indifferent sizes to accommodate different wafer sizes, and are made outof Teflon™ or an equivalent material.

The spacer ring 120 is placed in the bottom of the appropriate waferholder 124, then a semiconductor wafer can be placed on top, and anotherspacer ring 120 can then be put in place so that another semiconductorwafer can be added. This method of suspended stacking allows multiplewafers to be successfully processed during one run.

Since semiconductor wafers come in several different sizes, there areseveral different size wafer holders 124. When a smaller wafer holder124 is needed to process a semiconductor wafer, a chamber insert 121 isused to hold the wafer holder 124 in place and reduce the amount oftransitional fluid used. For example, if the drying chamber 40 has a sixand one half inch internal diameter and the operator wishes to process afour inch wafer, then a chamber insert 121 is put into place (having asix inch outer diameter with cavity 123 that has a four inch internaldiameter). The wafer then is placed in the four-inch wafer holder 124that is placed inside the chamber insert 121.

The wafer holder 124 is comprised of a body 125 and a handle 128. Thebody 125 has a plurality of fluid holes 127 that allow the transitionalfluid to reach the semiconductor wafers stacked inside the body 125.

Referring to FIG. 14, as described above, these software instructionscan be resident on the microprocessor 100 or stored on a separate memory101. At Step S1000, the computer system 99 executes softwareinstructions to cool the drying chamber 40 to a first chambertemperature. At S1100, the computer system 99 executes softwareinstructions to fill the drying chamber 40 with a transitional fluidhaving a critical point temperature and critical point pressure whilemaintaining the drying chamber 40 at the first chamber temperature, andthen the computer system 99 executes software instructions to purge theintermediary fluid from the drying chamber 40. At S1200, the computersystem 99 executes software instructions to fill to ensure the dryingchamber 40 is completely filled. At S1300, the computer system 99executes software instructions to activate a heater 32 to raise thetransitional fluid to its critical point pressure and critical pointtemperature, thereby reaching critical point equilibrium. At S1400, thecomputer system 99 executes software instructions to maintain thetransitional fluid at the critical point equilibrium for a second timeperiod. At S1500, the computer system 99 executes software instructionsto bleed the transitional fluid from the drying chamber 40 whilemaintaining the drying chamber 40 at the second chamber temperature andallowing the drying chamber pressure to drop. At S 1600, the computersystem 99 executes software instructions to vent the transitional fluidfrom the drying chamber.

The software instructions adapted to enable the computer system 99 tocontrol the drying chamber 40 and the associated valve assemblies willnow be described in greater detail.

Referring to FIG. 15A, if commanded to by the operator, the computersystem 99 executes diagnostic routines to determine if the criticalpoint drying apparatus 1 is in working order. These diagnostics aredescribed in greater detail below.

Next, the software instructions executed by the computer system 99 coolthe drying chamber 40 to a predetermined value. As described above, atS1010, the computer system 99 commands the cool valve 55 to open,thereby allowing the cooling fluid to flow from the inlet port 52,through the cool valve 55 and into the drying chamber walls, therebycooling the drying chamber 40 to the desired temperature level. Asdescribed above, by commanding the cool valve 55 to open, a coolingfluid flows around the drying chamber and evaporates in a heat exchangerelationship with the drying chamber 40. At S1020-1040, a determinationis made whether the drying chamber 40 has reached the desiredtemperature. If not, the cool valve remains open.

After the specimen has been placed in the drying chamber 40 immersed inan intermediary fluid and the chamber cover 75 secured, at S1110, thecomputer system 99 commands the fill valve 56 to open, thereby allowingthe transitional fluid to flow into the interior of the drying chamber40. This is known as the fill mode and the computer system 99 allows thetransitional fluid to flow into the drying chamber 40 for a presetamount of time. At the expiration of the preset time period, thecomputer system 99 commands the fill valve 56 to close. At S1115 toS1120, a determination is made if the preset time period has expired. Ifnot, the fill valve remains open.

Referring to FIG. 15B, at S1125 to S1135, an additional determination ismade whether the drying chamber 40 has remained at the desiredtemperature. If not, the cool valve is opened to cool the drying chamber40.

Next, at S1140, the computer system 99 executes the softwareinstructions for purging the intermediary fluid from the drying chamber40. This is known as the purge mode, such that the transitional fluidcompletely fills the drying chamber 40 and purges the intermediary fluidfrom the treated specimen in the drying chamber 40.

Referring to FIG. 15C, as shown by S1145 to S1150, the purging of thedehydrating fluid is controlled over a predetermined time period. Asshown by S1155 to S1165, while the drying chamber 40 is being filled andpurged, the computer system 99 maintains the drying chamber 40 at thepredetermined temperature by controlling the cool valve 55. Preferably,the predetermined temperature is 0° C.

Referring to FIG. 15B, when the computer system 99 has determined thatthe purging cycle has reached the end of its time period, at S1170, thefill valve 56, the purge valve 63 and the cool valve 55 are closed. Thedrying chamber 40 now should be completely filled by the transitionalfluid.

Referring to FIG. 15D, to ensure that the drying chamber 40 iscompletely filled, at S1210 to S1270, the computer system 99 executessoftware instructions that cycle back into the fill mode following thepurging of the intermediary fluid. The cycling back into the fill modeensures that the drying chamber 40 is completely filled by thetransitional fluid. Therefore, the fill valve 56 is reopened by thecomputer system 99 to facilitate this task. During this fill, as shownby S1240 to S1260, the computer system 99 monitors the temperature andcan activate the cool valve 55 if the drying chamber 40 needs to becooled. Once complete, at S1270, all the valves are commanded to beclosed.

Referring to FIG. 15E, at S1310 to S1340, after the computer system 99has completed the second chamber fill, the drying chamber 40 is heatedto pressurize the transitional fluid to its critical point as well asraise its temperature to the critical point. The computer system 99executes software instructions to activate the heater 33 mounted in thewall of the drying chamber. The computer system 99 monitors thetemperature to ensure that the chamber temperature does not exceed apreset limit. Preferably, this temperature limit is between 48 and 500°C.

The software instructions also comprise instructions that command thecomputer system 99 to indicate to the operator that the pressure andtemperature are above the critical point equilibrium. Preferably, thecomputer system 99 causes the heat LED 19 to flash thereby indicatingthat the specimens in the drying chamber 40 have reached the criticalpoint equilibrium. The critical point equilibrium is maintained for aprogrammed amount of time. At S1320, if the heat thermostatic sensor 30and the high pressure sensor 67 indicate that the critical pointequilibrium has not been reached, the software instructions command thecomputer system 99 to indicate to the operator that there is a problemwith the critical point drying sequence. At S1330, the computer system99 will flash all the operation indication LEDs to indicate a problemwith the critical point drying sequence. The operator is then allowed topress a switch to return to an earlier stage in the process at whichpoint the computer system 99 will then take over and complete the dryingsequence.

At S1410 to S1420, the computer system 99 executes software instructionsto determine if the specimen has been at critical point equilibrium fora predetermined time period.

Referring to FIG. 15F, at S1510 to S1520, once the programmed amount oftime at the critical point equilibrium has expired, the computer system99 executes software instructions to bleed off the pressure in thedrying chamber. The computer system 99 opens the bleed valve 62 andcontrols the bleeding of the pressure in the drying chamber 40 downbetween 100 and 600 psi while maintaining drying chamber temperatureabove 31° C. or above. Preferably, the low pressure threshold is 400psi. When the computer system 99 is executing the bleed mode, thecomputer system 99 controls the bleeding of pressure from the dryingchamber 40 in an even fashion. In dropping from 1100 psi to 400 psi, thecomputer system 99 allows the pressure to vent slowly from the dryingchamber 40. In addition, the computer system 99 commands the heater 33disposed on the drying chamber 40 to maintain the temperature of thedrying chamber 40 at 31° C. or above.

For the three valve embodiments illustrated in FIGS. 9A and 10A, atS1510, once the programmed amount of time at the critical pointequilibrium has expired, the computer system 99 executes softwareinstructions to bleed off the pressure in the drying chamber. Thecomputer system 99 opens the purge valve 63 and controls the bleeding ofthe pressure in the drying chamber 40 down between 100 and 600 psi whilemaintaining drying chamber temperature above 31° C. or above.Preferably, the low pressure threshold is 400 psi. When the computersystem 99 is executing the bleed mode, the computer system 99 controlsthe bleeding of pressure from the drying chamber 40 in an even fashion.In dropping from 1100psi to 400 psi, the computer system 99 allows thepressure to vent slowly from the drying chamber 40. In addition, thecomputer system 99 commands the heater 33 disposed on the drying chamber40 to maintain the temperature of the drying chamber 40 at 31° C. orabove.

At S1610 to S1620, when the drying chamber pressure reaches 400 psihowever, the software instructions command the computer system 99 toshut off the heaters and vent the remaining pressurized gas from insidethe drying chamber 40 directly out the vent line. The purge valve 63 isopened and the bleed valve 62 is closed. The purge valve 63 will remainopen until another run is commenced or the critical point dryingapparatus 1 is powered down.

For the three valve embodiments illustrated in FIGS. 9A and 10A, atS1610 to S 1620, when the drying chamber pressure reaches 400 psihowever, the software instructions command the computer system 99 toshut off the heaters and vent the remaining pressurized gas from insidethe drying chamber 40 directly out the vent line. The purge valve 63will remain open until another run is commenced or the critical pointdrying apparatus 1 is powered down.

Referring to FIG. 16, a tabular summary of the operational modes of thecritical point drying apparatus 1 are shown. For the three valveembodiments depicted in FIGS. 9A and 10A, the purge valve 63 alsoperforms the functions of the bleed valve as well.

In another aspect of the present invention, the predetermined operationsof the software instructions comprise instructions for conductingdiagnostic testing of several sensors used to control the critical pointdrying sequence. The software instructions test the high pressure sensor67 (or critical point pressure sensor), the low pressure sensor 68, theheat sensor 31 and cool sensor 32. The computer system 99 executing thesoftware instructions indicates on the display if each of theabove-identified sensors is operational. Preferably, the computer system99 lights an operation indication LED that indicates to the operatorthat the sensor is working properly.

The predetermined operations of the software instructions also comprisediagnostic instructions that support the testing of switches, theircorresponding operation indication LED switches and microprocessorinterrupt service routine. For example, to test the purge LED switch 18,pressing the purge LED switch 18 while the critical point dryingapparatus 1 is powered up will cause the purge LED switch 18 to light ifthe computer system 99 is executing the software correctly. In anotherexample, pressing the heat LED switch 19 when powering up the apparatuscauses the operation indication LEDs to individually light in sequence,thereby indicating that the microprocessor clock is operating correctly.Preferably, the operation indication LEDs light sequentially infifteen-second intervals.

The predetermined operations of the software instructions also comprisecalibration instructions for the cool sensor 32. The computer system 99will cool the drying chamber 40 independently of the cool sensor 32. Thefill LED switch 17 will indicate the status of the cool sensor 32 andwill allow the operator to determine if the cool sensor 32 is operatingcorrectly or that it needs adjustment.

The predetermined operations of the software instructions also compriseinstructions to ensure that the buildup of static electricity does notaffect the operation indication LED switches. The computer system 99constantly checks the status of the operation indication LED switches toensure that they are operating properly. If an operation indication LEDhas prematurely switched off, the computer system 99 determines at whatpoint of the critical point drying sequence the critical point dryingapparatus 1 is in, and then switches on the appropriate operationindication LED.

Another aspect of the present invention will now be discussed. Thepresent invention may be embodied on a computer program product forenabling a computer system to perform critical point drying techniqueswhen coupled to a critical point drying apparatus. The softwareinstructions that enable the computer system to perform predeterminedoperations as required by the present invention are borne on a computerreadable medium. The predetermined operations borne on the computerprogram product comprise software instructions for cooling the dryingchamber to a first chamber temperature. Preferably, the first chambertemperature is between 5 and —10° C.

The predetermined operations borne on the computer program productfurther comprise software instructions for filling the drying chamber 40with a transitional fluid having a critical point temperature andcritical point pressure while maintaining the drying chamber 40 at thefirst chamber temperature such that the transitional fluid completelydisplaces the intermediary fluid within a first time period. As notedabove, the first chamber temperature is preferably between 5 and —10° C.The predetermined operations borne on the computer program productmaintains the drying chamber 40 at the first chamber temperature whilethe intermediary fluid is exhausted from the interior of the dryingchamber 40. When the intermediary fluid is purged, the predeterminedoperations borne on the computer program product ensure that the dryingchamber is completely filled with transitional fluid to ensure asuccessful drying cycle. This is done by going through a quick fillcycle.

The predetermined operations borne on the computer program productfurther comprise software instructions for activating at least oneheater to raise the transitional fluid to its critical point pressureand critical point temperature, thereby reaching a critical pointequilibrium. Preferably, the predetermined operations borne on thecomputer program product command a heater 32 on the drying chamber 40 toheat the transitional fluid to at least 31° C. or above. Thepredetermined operations borne on the computer program product alsonotify the operator if the critical point equilibrium was successfullyreached.

The predetermined operations borne on the computer program productfurther comprise software instructions for maintaining the transitionalfluid at the critical point equilibrium for a second time period. Thepredetermined operations borne on the computer program product notifythe operator that the transitional fluid in the drying chamber is at itscritical point equilibrium.

The predetermined operations borne on the computer program productfurther comprise software instructions for bleeding the transitionalfluid from the drying chamber 40 while maintaining the drying chamber 40at the second chamber temperature. Preferably, the predeterminedoperations borne on the computer program product command the at leastone heater 32 to maintain a drying chamber temperature of at least 31°C. while the transitional fluid is bled from the drying chamber 40.

Another aspect of the present invention will now be discussed. Thepresent invention may be embodied on an article of manufacture, whichcomprises a computer readable medium having stored therein a computerprogram to control a drying chamber during a critical point dryingprocess. The article of manufacture comprises a computer program productthat bears a first computer code segment which, when executed on acomputer, cools the drying chamber 40 to a first chamber temperature.Preferably, the first chamber temperature is between 5 and −100° C.

The article of manufacture further comprises a second computer codesegment which, when executed on a computer, fills the drying chamber 40with a transitional fluid having a critical point temperature andcritical point pressure while maintaining the drying chamber 40 at thefirst chamber temperature such that the transitional fluid completelydisplaces the intermediary fluid within a first time period. As notedabove, the first chamber temperature is preferably between 5 and −10° C.The second computer code segment, when executed on a computer, maintainsthe drying chamber 40 at the first chamber temperature while theintermediary fluid is exhausted from the interior of the drying chamber.When the intermediary fluid is purged, the second computer code segment,when executed on a computer, ensures that the drying chamber 40 iscompletely filled with transitional fluid to ensure a successful dryingcycle.

The article of manufacture also comprises a third computer code segmentwhich, when executed on a computer, activates at least one heater toraise the transitional fluid to its critical point pressure and criticalpoint temperature, thereby reaching a critical point equilibrium.Preferably, the third computer code segment commands a heater 32 on thedrying chamber 40 to heat the drying chamber 40 to at least 31° C. orabove. The third computer code segment, when executed on a computer,also notifies the operator if the critical point equilibrium wassuccessfully reached.

The article of manufacture further comprises a fourth computer codesegment which, when executed on a computer, maintains transitional fluidat the critical point equilibrium for a second time period. The fourthcomputer code segment, when executed on a computer, notifies theoperator that the transitional fluid in the drying chamber is at itscritical point equilibrium.

Finally, the article of manufacture further comprises a fifth computercode segment which, when executed on a computer, bleeds the transitionalfluid from the drying chamber 40 while maintaining the drying chamber 40at the second chamber temperature. Preferably, the fifth computer codesegment, when executed on a computer, commands the at least one heater32 to maintain a drying chamber temperature of at least 31° C. while thetransitional fluid is bled from the drying chamber 40.

The foregoing description of the preferred embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit the presentinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of the present invention. The embodiments werechosen and described in order to explain the principles of the presentinvention and its practical application to enable one skilled in the artto utilize the present invention in various embodiments and with variousmodifications as are suited to the particular use contemplated.

Thus, while only certain embodiments of the present invention have beenspecifically described herein, it will be apparent that numerousmodifications may be made thereto without departing from the spirit andscope of the resent invention. Further, acronyms are used merely toenhance the readability of the specification and claims. It should benoted that these acronyms are not intended to lessen the generality ofthe terms used and they should not be construed to restrict the scope ofthe claims to the embodiments described therein.

1. A critical point drying apparatus for drying specimens, comprising: adrying chamber having concave end portions and at least one heater; afirst valve assembly that supplies a cooling fluid to the dryingchamber; a second valve assembly that supplies a transitional fluidhaving a critical point temperature and critical point pressure to thedrying chamber; a third valve assembly that allows an intermediary fluidto be purged from the drying chamber; a fourth valve assembly thatbleeds the transitional fluid from the drying chamber; and a computersystem that operates the first, second, third and fourth valveassemblies and that activates the at least one heater to heat thetransitional fluid above the critical point temperature and topressurize the transitional fluid above the critical point pressure. 2.The critical point drying apparatus as claimed in claim 1, wherein thethird and fourth valve assemblies comprise heaters.
 3. The criticalpoint drying apparatus as claimed in claim 2, wherein the third andfourth valve assemblies further comprise check valves to prevent thebackflow of intermediary fluid into the drying chamber.
 4. The criticalpoint drying apparatus as claimed in claim 1, further comprising aheated pressure relief valve.
 5. The critical point drying apparatus asclaimed in claim 1, wherein the computer system comprises a plurality ofrelays to control the first, second, third and fourth valve assemblies.6. The critical point drying apparatus as claimed in claim 1, whereinthe drying chamber is adiabatically cooled.
 7. The critical point dryingapparatus as claimed in claim 1, wherein the drying chamber has at leastone inlet port for the introduction of the transitional fluid into theinterior of the drying chamber.
 8. The critical point drying apparatusas claimed in claim 7, wherein the at least one inlet port is angledwith respect to an inner surface of the drying chamber.
 9. The criticalpoint drying apparatus as claimed in claim 1, wherein the drying chamberfurther comprises mounting posts, wherein the mounting posts are pinnedto a sidewall of the drying chamber.
 10. The critical point dryingapparatus as claimed in claim 1, further comprising a condensercollector that collects intermediary fluid after exiting the third valveassembly.
 11. The critical point drying apparatus as claimed in claim10, wherein the condenser collector comprises a reservoir for heatingfrozen intermediary fluid.
 12. The critical point drying apparatus asclaimed in claim 1, wherein the critical point drying apparatus receivescooling fluid from a closed loop refrigeration system.
 13. The criticalpoint drying apparatus as claimed in claim 1, wherein the drying chamberfurther comprises a viewing port.
 14. The critical point dryer apparatusas claimed in claim 1, wherein the drying chamber further comprises atleast one wafer holder and at least one spacer ring.
 15. The criticalpoint dryer apparatus as claimed in claim 14, wherein the drying chamberfurther comprises at least one chamber insert.
 16. A critical pointdryer apparatus, comprising: a drying chamber having concave endportions and at least one heater; and a computer system adapted tocontrol the drying chamber during a critical point drying process, thecomputer system comprising: a processor; a memory comprising softwareinstructions adapted to enable the computer system to: cool the dryingchamber to a first chamber temperature; fill the drying chamber with atransitional fluid having a critical point temperature and criticalpoint pressure while maintaining the drying chamber at the first chambertemperature such that the transitional fluid completely displaces anintermediary fluid within a first time period; activate at least oneheater to raise the transitional fluid to its critical point pressureand critical point temperature, thereby reaching a critical pointequilibrium; maintain the transitional fluid at the critical pointequilibrium for a second time period; and bleed the transitional fluidfrom the drying chamber while maintaining the drying chamber at a secondchamber temperature and allowing the drying chamber pressure to drop.17. The critical point dryer apparatus according to claim 16, whereinthe software instructions that fill the drying chamber with transitionalfluid are further adapted to fill the drying chamber completely withtransitional fluid, by means of a post purge fill cycle.
 18. Thecritical point dryer apparatus according to claim 16, wherein thesoftware instructions that activate at least one heater are furtheradapted to command the computer system to heat the drying chamber to atleast 31° C.
 19. The critical point dryer apparatus according to claim16, wherein the software instructions that activate the at least oneheater are further adapted to command the computer system to pressurizethe drying chamber pressure to at least 1175 pounds per square inch. 20.The critical point dryer apparatus according to claim 16, wherein thesoftware instructions are further adapted to enable the computer systemto perform diagnostic testing on the critical point dryer apparatus. 21.The critical point dryer apparatus according to claim 20, wherein thesoftware instructions are further adapted to enable the computer systemto perform diagnostic testing of a sensor mounted on the drying chamber.22. The critical point dryer apparatus according to claim 16, whereinthe software instructions are further adapted to enable the computersystem to signal to an operator that the temperature and the pressure ofthe transitional fluid are above the critical point equilibrium.
 23. Thecritical point dryer apparatus according to claim 22, wherein thesoftware instructions are further adapted to enable the computer systemto signal to the operator that the transitional fluid has not reachedcritical point equilibrium.
 24. The critical point dryer apparatusaccording to claim 16, wherein the software instructions that bleed thetransitional fluid from the drying chamber are further adapted tocommand the computer system to shut off the at least one heater when thedrying chamber pressure drops between 600 and 100 pounds per squareinch.
 25. The critical point dryer apparatus according to claim 24,wherein the software instructions that bleed the transitional fluid fromthe drying chamber are further adapted to command the computer system tovent the transitional fluid when the drying chamber pressure drops below400 pounds per square inch.
 26. A critical point drying apparatus fordrying specimens, comprising: a drying chamber having at least oneheater and further comprising at least one wafer holder; a first valveassembly that supplies a cooling fluid to the drying chamber; a secondvalve assembly that supplies a transitional fluid having a criticalpoint temperature and critical point pressure to the drying chamber; athird valve assembly that allows an intermediary fluid to be purged fromthe drying chamber, and that bleeds the transitional fluid from thedrying chamber; and a computer system that operates the first, secondand third valve assemblies and that activates the at least one heater toheat the transitional fluid above the critical point temperature and topressurize the transitional fluid above the critical point pressure. 27.The critical point dryer apparatus as claimed in claim 26, wherein thedrying chamber further comprises at least one chamber insert.
 28. Acritical point drying apparatus for drying specimens, comprising: adrying chamber having concave end portions and at least one heater; afirst valve assembly that supplies a cooling fluid to the dryingchamber; a second valve assembly that supplies a transitional fluidhaving a critical point temperature and critical point pressure to thedrying chamber; a third valve assembly that allows an intermediary fluidto be purged from the drying chamber, and that bleeds the transitionalfluid from the drying chamber; and a computer system that operates thefirst, second and third valve assemblies and that activates the at leastone heater to heat the transitional fluid above the critical pointtemperature and to pressurize the transitional fluid above the criticalpoint pressure.
 29. The critical point dryer apparatus as claimed inclaim 28, wherein the drying chamber further comprises at least onewafer holder.
 30. The critical point dryer apparatus as claimed in claim28, wherein the drying chamber further comprises at least one chamberinsert.